Following a lot of hype—and a quarter-century of work—”Now and Then,” presumably the last song to feature all four original Beatles, is here. The track dropped Thursday and the music video, directed by Peter Jackson, hit YouTube on Friday. Sweet and haunting, it’s full of piano and strings, and it wouldn’t have been possible without the machine learning technology Jackson used on the docuseries Get Back.

How the AI technology came to be the thing that saved the song is a bit of a journey. Years after John Lennon died in 1980, his wife, the musician and multimedia artist Yoko Ono, told his bandmate Paul McCartney that she had a demo tape Lennon had recorded at their apartment in the Dakota in New York City.

In the 1990s, when the three remaining Beatles—McCartney, Ringo Starr, and George Harrison—were working on recordings for their Anthology records, they tried to salvage “Now and Then” from an old cassette. At the time, though, Lennon’s vocals were too awash in the sounds of the piano he was playing and the technology didn’t exist to extract them. “‘Now and Then’ just kind of languished,” McCartney says in a new short documentary about the song.

Harrison died in 2001 and it seemed the song might languish forever. Then, in 2022, as Jackson was working on Get Back, a documentary made from 1969 footage of the band making the album/concert/film that would become Let It Be, he and his team developed AI technology that allowed him to separate out all of the various instruments and voices in the recordings. “We thought, ‘Well, we’d better send John’s voice to them, off of the original cassette,” McCartney says.